It's going to depend a lot on the school and the area you are in. In general, it is more difficult to get into a CRNA program than other grad programs because there are more students applying than there are spaces available. Also, the programs are very demanding and the schools look at past academic records very hard as an indicator for probable success.
Check out this website for more info. It's very comprehensive.
http://www.anesthesia-nursing.com/index.html